Pre-cast concrete manholes are widely used throughout the world to provide access to storm and sanitary sewer systems and a variety of other below-ground installations of piping, cables and the like. If such access is to be used, a ladder or steps must be provided to permit workers to descend into and climb out of the manhole. A variety of structures have been employed to make this possible, including ladders temporarily positioned in the manholes and ladders or steps permanently attached to the inside wall of the manholes.
One highly successful such step is disclosed in U. S. Pat. No. 4,100,997, which is incorporated herein by reference. U. S. Pat. No. 4,100,997 describes a steel reinforced plastic step that is injection molded of copolymer polypropylene. Support members that terminate in ends having molded, deformable annular projections are driven into pre-formed holes in the inside wall of the manhole.
In order to use a step of the type described in U. S. Pat. No. 4,100,977, holes must be formed in the inside face of the manhole having the proper size, location, orientation and spacing to receive the step legs. This has conventionally been accomplished by inserting metal rods or other forms from inside the form through appropriately located holes in the inner wall of the manhole form, so that properly shaped and located holes will be formed in the concrete manhole. Such hole-forming rods must also be withdrawn from inside the form before the formed concrete manhole and the form are separated. Doing so is time-consuming and arduous because it typically requires that a worker climb inside of the inner manhole form, which is difficult and unpleasant to do. Accordingly there is a need for a better method of forming holes in pre-cast manholes to receive steps.
Manholes frequently are manufactured with a plastic liner that forms the inside of the manhole and protects the concrete from chemicals that potentially would damage the concrete. Manufacture of manholes with such liners presents additional challenges to the provision of steps despite the presence of the liner and without breaching the protective layer it provides. For example, step holes may be drilled into the formed manhole, but some form of liner or other sealant must then be inserted into the step hole. Alternatively, the liner may be manufactured with deep indentations which act as step holes; however, such indentations may substantially increase the cost of the mold used to form the liner.